The Study On The Hydrogen Donation Capability Of Supercritical Water

Water near or above its critical point is attracting more attention andwidely studied because of its particular physical and chemical properties.Recently supercritical water has been utilized by many researchers toprocess carbonaceous feed stocks and recycle waste plastics, which cannot be refined by available commercial processes, and found thatsupercritical water can prevent the coke formation and promote the oilyield compared to pyrolysis in inert atmosphere. The effect ofsupercritical water was attributed by some researchers to the free racialmechanism which proposes that supercritical water is involved in thereaction by the formation of H· and HO· radicals, while ionic mechanismand physical effect were suggested by other researchers leading to acontroversy over the role of supercritical water. Until now the role ofsupercritical water is still not clarified.To verify the free radical mechanism, in this paper three probecompounds, p-benzoquinone, naphthalene and azobenzene, are studied insupercritical water, high temperature water, N2atmosphere with andwithout tetralin as hydrogen donor. The results show that: (1) Supercritical water has limited ability to generate H· andHO· radicals that are capable to “hydrogenate” the hydrocarbons.(2) Some of the reaction behaviors in supercritical water may beattributed to presence of residual O2in the system which results inoxidation of hydrocarbons to CO, followed by water-gas-shift reaction forH2, and hydrogenation of hydrocarbons.(3) Free radicals generated by bond cleavage in hydrocarbons canhardly capture the hydrogen from supercritical water and couple withHO· radical forming oxygenated chemicals which may be attributed toionic reaction behavior in supercritical water.(4) The diversity of transportation properties between inert gases andsupercritical water may result in the experimental gaps of carbonaceousmaterials pyrolysis in inert atmosphere and supercritical water.